1. Field of the Invention
The present invention relates to a method and apparatus for preparing a slurry for a Chemical Mechanical Polishing (CMP) apparatus.
2. Description of the Related Art
As semiconductor integrated circuit design rules shrink, CMP has gained popularity in processes for planarizing layers such as interlayer films. Most of slurries used for CMP comprise a solid-liquid dispersion system in which fine particles are dispersed in an aqueous solution with reagents such as a pH adjuster. The slurries are available from a variety of slurry manufactures as a stock slurry solution in the form of liquid. A small amount (e.g., on the order of 2% by weight in the resultant solution) of hydrogen peroxide (H2O2) is a common additive for the stock slurry solution. Thus, the stock slurry solution and the additive and the like are materials for the slurry.
A common apparatus for preparing a slurry for a polishing apparatus is of a type shown in a cross-section in FIG. 2. The slurry preparation apparatus can also function as a slurry tank (slurry feeder) for supplying the slurry.
In FIG. 2, the slurry preparation apparatus 1 has pipes 2, 3 and 4 inserted into a bath through an upper lid. A pipe 5 is connected to the bottom of the bath. The pipe 2 is for supplying the stock slurry solution (as it is purchased from a manufacturer) and the pipe 3 for supplying an additive. The pipe 5 is for drawing and supplying the slurry to a slurry feeder, and the pipe 4 for returning the slurry back into the bath.
The pipes 4 and 5 are connected to a CMP apparatus, and used for supplying the slurry from the slurry feeder 1 to the CMP apparatus while circulating the slurry. A pipe 6 is connected between the pipes 4 and 5, and serves as a bypass between the pipes 4 and 5 that circulate the slurry between the slurry feeder 1 and the CMP apparatus. Additionally, a pump 7 and valves 8 and 9 are provided as auxiliary equipment.
The preparation of the slurry is accomplished by supplying the stock slurry solution and the additive through the pipes 2 and 3, respectively, and stirring them with a stirrer, not shown. Similarly, the stock slurry solution and the additive are supplied and stirred to prepare the slurry as it is lessened in the slurry feeder 1. In the operation, the amount of the stock slurry solution and the additive supplied are managed using level gauges (not shown) disposed within the slurry feeder 1.
In some instances, a slurry is not initially prepared in the slurry feeder 1 as described above, and an externally and separately prepared slurry is supplied. In this case, the pipe 2 and/or the pipe 3 are used only to add the stock slurry solution and/or additive.
In preparing a slurry in the above-described conventional slurry feeder 1, however, there arises a problem that the H2O2 concentration may decrease over time due to a resolution nature of H2O2, even though the initial amount of the stock slurry solution and additive, H2O2, supplied could accurately be managed. As a result, the mixture ratio varies and it would seriously affect a polishing quality.
To avoid the problem, it has been a common practice to regularly sample a slurry, measure the H2O2 concentration in the slurry, and feed the tank either with an additional slurry prepared in an external tank with higher H2O2 concentration than a baseline such that the current H2O2 concentration in the slurry can be corrected, or with a stock slurry solution and/or H2O2 using the pipe 2 and/or the pipe 3.
However, the conventional tasks above burden workers: the current amount of slurry and the H2O2 concentration in the slurry should be accurately obtained before an additional amount of slurry and the H2O2 concentration in the additional slurry are calculated, and an additional slurry needs to be prepared, or a stock slurry solution and/or H2O2 needs to be supplied using the pipe 2 and/or the pipe 3 according to the calculations. These require manual operation that is not suitable to a process control.
These also require facilities such as equipment for sampling, external tanks for an additional slurry, and the like.